The hard carapace on the crab can protect the crab from natural enemies, but the carapace does not expand as the body grows. Therefore, crab growth is episodic, that is, after a period of time, the body will continue to grow after the old shell is shed. The largest crabs on earth are spider crabs, with their legs spread up to 3.7 meters wide. The smallest crabs are bean crabs, less than half a centimeter in diameter.

Although crabs are small, they have all five internal organs. After removing the hard shell of the crab, we can find that the crab's body is protected by a shell. These shield-like shells are called carapace by biologists. The body of the crab is symmetrical and can be divided into the forehead area, eye area, heart area, liver area, stomach area, intestine area and gill area. There are appendages on both sides of the crab's body. The appendages on the head are called antennae, which provide touch and smell functions. Some appendages have mouth functions, which are used to tear food apart and bring it into the mouth.

The crab's chest has five pairs of appendages called thorax. The pair of appendages in the front are equipped with strong claws, which can be used for foraging. The remaining four pairs of appendages are the crab's feet. Crabs rely on these four pairs of appendages to move and move. Their walking appearance is unique. And interestingly, most of them are walking sideways instead of going straight forward. The exception is monk crabs, which walk straight.

Crabs spend most of their time looking for food. They are not picky eaters and will eat anything their pincers can get. Small fish and shrimp are their favorite, but some crabs eat seaweed and even animal carcasses or plants.

Crabs eat other animals, and other animals may also eat crabs. For example, humans regard crabs as delicacies. There are also waterfowl that eat crabs! Some fish also like to eat crab legs just like humans. When young and immature crabs float in groups in the sea, they may be gobbled up by other marine life. Therefore, crabs must lay many eggs when they spawn.